Minutes later, two finalists to be Mr. Trump’s running mate began weighing in. Newt Gingrich proposed a loyalty test for American citizens who are Muslim and deporting those who believe in Shariah law, while Gov. Mike Pence of Indiana, referring to terrorism, called for America to “defeat this enemy of civilization at its source.”

Yet for all the bellicose proclamations, Mr. Trump and his allies would be sharply constrained — by lawmakers, foreign allies, treaties and the Constitution — in achieving these goals, and Mr. Trump is not much closer to providing specific proposals on national security than he was 13 months ago when he began running for president.

On Friday, as he announced Mr. Pence as his choice and prepared to claim the Republican nomination at the party’s convention next week, Mr. Trump still lacked a detailed foreign policy agenda and a deep bench of advisers, appearing instead like a man who had taken his cues about war from Fox News commentators and Twitter users.